1. Field of the Invention
The invention relates to a method of producing a SiC single crystal using a solution technique.
2. Description of the Related Art
SiC has an energy band gap that is wider than that of Si. Therefore, Various techniques have been proposed for producing a high-quality SiC single crystal that is suitable for being used as, for example, a semiconductor material. Various methods of producing the SiC single crystal have been attempted. Among them, a sublimation technique and a solution technique are most commonly used at present. When using the sublimation technique, a growth rate is high. However, a defect, such as a micropipe, is likely to occur, and a crystal polymorph is likely to be formed. In contrast, when using the solution technique, the defect, such as the micropipe, is not likely to occur, and the crystal polymorph is not likely to be formed, although the growth rate is low. Therefore, the solution technique is considered to be a promising technique.
In a method of producing a SiC single crystal using the solution technique, a crucible made of a material containing carbon (graphite in general) is used. A material containing silicon is molten in the crucible. In melt containing silicon in the crucible, a temperature gradient is maintained so that the temperature of the melt decreases from an inner portion of the melt toward a surface of the melt. Carbon is dissolved from a high-temperature lower portion of the crucible into the melt (i.e., a solvent) containing silicon. Then, carbon is carried upward mainly by a convection flow of the melt. When the carbon reaches a low-temperature portion near the surface of the melt, the low-temperature portion is oversaturated. A seed crystal, which is fixed at an end of a seed crystal fixing shaft, is brought into contact with the surface of the solution produced by dissolving carbon into the melt containing silicon. Thus, the SiC single crystal is grown on a lower surface (i.e., a solution contact surface) of the seed crystal.
However, various technical problems need to be solved to produce a high-quality SiC crystal with a large area at a high growth rate using the solution-growth technique.
For example, a technical problem that needs to be solved to produce a high-quality SiC crystal is described in Materials Science and Engineering, B61-62 (1999) 29-39. In the publication, it is described that when a SiC single crystal is grown using a solution containing an Si solvent, inclusion of Si is caused in the SiC crystal. The inclusion is a collective term for phases that exist in the SiC single crystal, and that are different from the SiC single crystal. That is, the term “the inclusion” signifies heterophases mixed into the SiC single crystal. Typical example of the inclusion is a particle derived from a droplet of Si or C. Examples of the inclusion include silicides, carbides, nitrides, and oxides. Examples of the inclusion further include a SiC crystal different from a given crystal polymorph, for example, a 3C—SiC crystal mixed into a 6H—SiC single crystal, and gas (air bubbles) confined in a crystal. The inclusion is caused by a non-uniform surface in a growth interface, that is, “morphological instability”. The non-uniform surface has a macrostep structure. It is considered that Si in a solvent enters an area between macrosteps, and Si is confined in the crystal due to the growth of the steps in a lateral direction.
Japanese Patent Application Publication No. 2006-117441 (JP-A-2006-117441) describes a method of producing a silicon carbide single crystal using the solution-growth technique, in order to provide a SiC single crystal production method that makes it possible to produce a high-quality SiC single crystal in which no inclusion exists, at a high rate. In the method, melt is agitated by periodically changing the rotational speed, or the rotational speed and the rotational direction of a crucible, that is, by a so-called Accelerated Crucible Rotation Technique (ACRT).
When the SiC single crystal was actually grown under a condition described in the publication No. 2006-117441. As a result, flatness of a growth surface of the produced single crystal varied, and the growth rate was not as high as expected.
In order to clarify the cause of the above-described situation, a device that visually simulates a flow of the solution in the crucible was developed, and the flow of the solution was analyzed using the device. As a result, it was found that an ideal upward flow was not generated at a center portion (refer to FIG. 1). That is, it was found that there was a high possibility that a swirl was retained around a seed crystal, and dissolved carbon was not efficiently carried onto the seed crystal.